The long-term objective of our research program is to understand transcriptional control of epidermal development and differentiation within the epidermis. One critical function of the epidermis is to form an effective permeability barrier;defective epidermal barrier contributes to many skin diseases. The formation of the barrier, which is intricately linked to the terminal differentiation of epidermal keratinocytes, depends on structural proteins, proteases, lipid composition and cell-cell adhesion in the granular layer of the epidermis. While we have gained significant understanding of the cell biology of the barrier, much less is known about the mechanisms that control its formation. In particular, we do not understand well the transcriptional control that underlies the coordinated expression multiple genes encoding barrier components. In the last funding period of this award, we discovered the Grainyhead-like transcription factor Grainyhead-like Epithelial Transactivator 1 (Get1/Grhl3) based on its interaction with the LIM-only factor LMO4. During development, both Get1 and LMO4 are expressed in the developing epidermis, suggesting a role in epidermal formation. Deletion of the Get1 gene leads to alterations in the expression of multiple genes linked to terminal differentiation and barrier formation of the epidermis. Furthermore, simultaneous deletion of both Get1 and LMO4 leads to a more severe defect in terminal differentiation, indicating that we have identified key transcription factors for control of terminal differentiation and barrier formation. Based on these findings we hypothesize that Get1 directly regulates multiple terminal differentiation genes in a stage-specific manner, and that Get1 forms a complex with LMO4 to regulate at least a subset of these genes. In this renewal application, we propose three Specific Aims: 1) to determine the mechanisms underlying deregulation of multiple terminal differentiation genes in Get1 knockout mice;2) to define the temporal requirement for Get1 in regulation of epidermal terminal differentiation;and 3) to determine whether Get1 and LMO4 act in a complex to regulate terminal differentiation of the epidermis. These investigations will enhance our understanding of defective epidermal barrier formation, and may aid the discovery of treatment options. Defective skin barrier is a significant public health problem that occurs in several inherited and acquired diseases of the skin, as well as in premature infants.